An EL display device has organic EL elements. In each organic EL element,
an organic thin film having a transparent electrode on one surface and a
metal electrode on the other surface is formed on a substrate, and
positive and negative carriers are injected from the transparent electrode
and the metal electrode, respectively, to emit light. The substrate is a
circuit board having a metal interconnection, and the metal electrode of
each organic EL element is formed at a predetermined portion on the upper
surface of the circuit board.

1. An EL display device having organic EL elements in each of which an
organic thin film having a transparent electrode on one surface and a
metal electrode on the other surface is formed on a substrate and positive
and negative carriers are injected from said transparent electrode and
said metal electrode, respectively, to emit light, wherein

said substrate is a circuit board having a metal interconnection and said
metal electrode of each of said organic EL elements is formed at a
predetermined portion on an upper surface of said circuit board, and
wherein radiation fins are mounted on said lower surface of said circuit
board, and heat of said organic EL elements is radiated from said
radiation fins.

2. A device according to claim 1, having a structure in which said metal
electrode for each of said organic EL elements is formed at a portion
other than on said metal interconnection of said circuit board and can be
connected to said metal interconnection of said circuit board.

3. A device according to claim 1, having a structure in which said organic
EL elements are arranged in a dot matrix of n rows.times.m columns or n
rows.times.n columns.

4. A device according to claim 3, comprising a via interconnection for
connecting said metal interconnection formed on said upper surface of said
circuit board to a lower surface of said circuit board so as to achieve
connection to each of said organic EL elements arranged on said upper
surface of said circuit board, and a metal interconnection formed on said
lower surface of said circuit board in order to connect said via
interconnection to a driver formed on said lower surface of said circuit
board.

5. An EL display device having organic EL elements in each of which an
organic thin film having a transparent electrode on one surface and a
metal electrode on the other surface is formed on a substrate and positive
and negative carriers are injected from said transparent electrode and
said metal electrode, respectively, to emit light, wherein

said substrate is a circuit board having a metal interconnection and said
metal electrode of each of said organic EL elements is formed at a
predetermined portion on an upper surface of said circuit board, and
wherein a cooling element is inserted in a connecting portion of said
metal electrode of each of said organic EL elements and said metal
interconnection of said circuit board.

6. A device according to claim 5, having a structure in which said metal
electrode for each of said organic EL elements is formed at a portion
other than on said metal interconnection of said circuit board and can be
connected to said metal interconnection of said circuit board.

7. A device according to claim 5, wherein said cooling element is a Peltier
effect element.

8. A device according to claim 5, having a structure in which said organic
EL elements are arranged in a dot matrix of n rows.times.m columns or n
rows.times.n columns.

9. A device according to claim 8, comprising a via interconnection for
connecting said metal interconnection formed on said upper surface of said
circuit board to a lower surface of said circuit board so as to achieve
connection to each of said organic EL elements arranged on said upper
surface of said circuit board, and a metal interconnection formed on said
lower surface of said circuit board in order to connect said via
interconnection to a driver formed on said lower surface of said circuit
board.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an EL display device using organic EL
elements and a method of manufacturing the same and, more particularly, to
an EL display device having an integrated structure of organic EL elements
and drivers for them, and a method of manufacturing the same.

2. Description of the Prior Art

Conventional organic EL elements will be briefly described with reference
to FIGS. 1 and 2.

Metal electrodes 14 that form anodes, an organic thin film 13, and metal
electrodes 12 that form cathodes are formed on a transparent substrate 11
to build organic EL elements. In this structure, positive and negative
carriers are injected from the negative electrodes 12 and positive
electrodes 14, respectively, to emit light toward the transparent
electrodes.

As a prior art for integrating organic EL elements and drivers for them to
form an EL display device, one is reported by C. C. Wu et al. of Princeton
University in "Integration of Organic LED's and Amorphous Si TFT's onto
Unbreakable Metal Foil Substrates", IEDM Tsch. Dig., 957-959, 1996 at
International Electron Device Meeting (IEDM 196) held in December, 1996.

The structure of the EL display device reported in this meeting is as shown
in FIG. 3. This EL display device is a single-layer device using a
stainless steel substrate 15. A Pt electrode 16 and a thin Ag electrode 18
having a thickness of 150 .ANG. are formed as the anode and cathode,
respectively, and a polyvinyl carbazole (PVK)-based polymer thin film 17
is formed as an organic film by spin coating. Emitted light is output to
the thin Ag electrode 18 side.

In the structure shown in FIG. 3, although the organic EL element and the
driver are integrated, the reported luminous efficiency of the organic EL
element is as small as about 0.01%, which is by no means comparable to the
luminous efficiency of 4% to 5% which is realized by a single organic EL
element. Therefore, this EL display device was not evaluated except that
it proposed an organic EL element on a new concept.

C. C. Wu et al. subsequently reported the structure of an EL display device
shown in FIG. 4 at International Symposium (SID 197) of SID (Society for
Information Display) held in May, 1997.

The structure shown in FIG. 4, regarding its stainless steel substrate 15,
a Pt electrode (anode) 16, and a polymer thin film 17, is identical to
that shown in FIG. 3 which is reported at IEDM '96. The difference of this
structure resides in that the cathode is improved by fabricating it as a
multilayer electrode consisting of a 150-.ANG. MgAg film and a 400-.ANG.
ITO film, e.g., an ITO/thin MgAg laminated electrode (cathode) 19.
According to the report, this improvement increased the luminous
efficiency by about 1%.

In the structures shown in FIGS. 3 and 4, emitted light is output through
the cathode. The structure shown in FIG. 3 uses an Ag film, which is in no
way transparent, as a cathode. The thickness of this Ag film is decreased
to obtain certain transparency. In the structure shown in FIG. 4, a
transparent ITO film is used to provide an improvement in this point.
However, the ITO film has an excessively large work function to make
electron injection difficult (disadvantageous) due to ionization potential
of the organic thin film. This difficulty is removed by interposing a thin
MgAg film. In this case, although the emission efficiency is improved,
since the MgAg film is by no means transparent, either, it decreases
transmittance all the same.

A still another prior art is described in Japanese Unexamined Patent
Publication No. 61-231584.

FIG. 5 is a perspective view showing the structure of an EL display device
described in Japanese Unexamined Patent Publication No. 61-231584.

In the structure shown in FIG. 5, inorganic EL elements using an inorganic
electroluminescent material, e.g., Zn:Mn, is formed on one major surface
of a ceramic substrate 21. In order to connect the inorganic EL elements
with drivers 27 integrally formed on the other surface of the ceramic
substrate 21, interconnections that are connected to the inorganic EL
elements on one side and to the drivers 27 on the other side are formed to
extend through the ceramic substrate 21. This realizes integration of the
inorganic EL elements and the drivers.

The structure shown in FIG. 5 is manufactured in the following manner.

As the inorganic EL elements of this EL display device, first electrodes 22
are formed on one major surface of the ceramic substrate 21, and
subsequently an insulating layer 23, an inorganic emission layer 24, an
insulating layer 25, and second electrodes 26 are sequentially formed.

Since emitted light is output to the second electrodes 26 side, transparent
electrodes are used as the second electrodes 26. The material that forms
the inorganic EL elements has high heat resistance. After the inorganic
emission layer 24 is formed, the transparent second electrodes 26 are
formed in accordance with ordinary sputtering.

Whereas the material that forms the inorganic EL elements has high heat
resistance, the material of the organic EL elements lacks heat resistance.
The difference in heat resistance of the material is one factor that has
interfered with integration of the organic EL elements and their drivers.

Although the structure shown in FIG. 5 and the structure according to the
present invention are similar in terms of integration, they are completely
different in that this integration is enabled for organic EL elements in
the present invention.

An inorganic EL element and an organic EL element have different emission
mechanisms. Accordingly, whereas the inorganic EL element requires a drive
voltage equal to or higher than 100 V, the organic EL element can be
driven with a voltage of equal to or lower than 10 V. The reason the
inorganic EL element requires a high drive voltage is that, unlike in the
organic EL element, the inorganic EL element is not excited by
recombination, but electrons collide against luminescent centers by
electric field acceleration to emit light. The organic EL element has
gained interest in terms of this drive voltage as well. However, the
material of the organic EL element lacks heat resistance, as described
above, and is conventionally difficult to integrate the organic EL element
with the driver.

The temperature must be maintained to be equal to or lower than 80.degree.
C. throughout the entire process of the manufacture of the organic EL
element, which is a very large limitation. For this reason, although the
various advantages of integration, e.g., down-sizing, weight reduction,
and cost reduction, are sufficiently recognized, conventionally, the
structure as an EL display device is barely realized by using a polymer
having comparatively high heat resistance among the organic EL element
materials and forming an ITO film in accordance with RF magnetron
sputtering by limiting the condition, as in the examples shown in FIGS. 3
and 4. When a polymer is compared with a low molecule-based organic EL
element material widely used as the organic EL element material, although
its heat resistance is better, it is not suited for vacuum deposition that
can achieve good film formation, and it can only be applied to spin
coating and the like.

Attempts have been constantly made to form a transparent electrode at low
temperatures. A typical prior art related to this is the technique
described in Japanese Unexamined Patent Publication No. 9-71860.

This technique is mainly based on the need for formation of an ITO
electrode on a plastic substrate at a low temperature, and provides an
improvement over the sputtering target. A target manufactured by mixing
indium oxide and zinc oxide in an oxide of an element having a valance
equal to or larger than +3 as needed, molding and sintering the mixture,
and annealing the sintered body, and a method of manufacturing the same
are known. As a practical example of the manufacture of a target, for
example, a case is reported wherein film formation is performed under the
conditions shown in Table 1 by using a target manufactured by mixing 254 g
of In.sub.2 O.sub.3 having a purity of 99.99% and an average particle
diameter of 1 .mu.m, 40 g of zinc oxide powder having a purity of 99.99%
and an average particle diameter of 1 .mu.m, and 6 g of titanium oxide
powder having a purity of 99.99% and an average particle diameter of 1
.mu.m.

In Table 1, the substrate temperature is room temperature. This probably
means, in the absence of specific description, that the substrate is not
heated or cooled particularly. During sputtering, the temperature of the
substrate naturally rises due to excessive energy of the film formation
particles.

The present invention has been made in consideration of the above situation
in the prior art, and has as its first object to provide a manufacturing
method in which the limitation on the process that, due to poor heat
resistance of the organic EL element material, an organic thin film can be
formed only after a transparent electrode is formed is overcome, so that
an organic thin film can be formed arbitrarily.

It is the second object of the present invention to eliminate conventional
restriction that the material of the substrate of the organic EL element
is limited to only a transparent material such as glass or plastic, so
that materials of a wide range can be used. In connection with this, it is
also an object of the present invention to provide a lightweight,
low-profile, compact EL display device in which organic EL elements and
drivers are integrated by using a printed wiring board made of, e.g., an
epoxy resin, which is widely used as a circuit board, as the substrate of
the organic EL elements.

In order to achieve the first object of the present invention, according to
the manufacturing method of the present invention, when forming a
transparent electrode (ITO thin film) on an organic thin film, a cooled
metal mask is placed in a sputtering particle flow so that unwanted
particles will not increase the temperature of the organic thin film. The
energy of the sputtering particles that reach the surface of the substrate
through a hole in the mask is suppressed to a minimum level, and some of
the sputtering particles are ionized and accelerated with an electric
field to replenish energy, thereby realizing stable film formation.

In order to achieve the second object of the present invention, according
to the EL display device of the present invention, an organic thin film
having a transparent electrode on one surface and a metal electrode on the
other surface is formed on a substrate. Since emitted light is externally
output through the transparent electrode, the substrate which is located
on the opposite side to the transparent electrode with respect to the
organic thin film and which is in contact with the organic thin film need
not be transparent. More specifically, the substrate material need not be
transparent, and substrate materials of a wide range can be used. As a
typical example, an EL display device can be realized in which organic EL
elements and drivers are integrated by forming the organic EL elements on
a printed wiring board such that the printed wiring board and the organic
EL elements are connected to each other. Integration provides effects such
as reductions in the material cost, weight, thickness, number of
processing steps, and size, that are achieved by reduction of the glass
substrate and connection flexible leads (FPC).

In the conventional manufacturing method, maintaining the substrate
temperature at room temperature means that substrate heating is not
performed, and the substrate temperature during film formation is
uncontrolled. According to the present invention, the substrate
temperature during film formation is positively controlled to prevent an
unwanted temperature increase.

The first effect obtained by the present invention is that a conventional
limitation on the process that an organic thin film can be formed only
after formation of a transparent electrode is eliminated. Accordingly,
transparency is not an indispensable condition for the substrate, and
organic EL elements can be arbitrarily formed on various types of
substrates.

The second effect obtained by the present invention is that, in connection
with the first effect, a circuit board such as a printed wiring board can
be used as the substrate, so that a driver-integrated organic EL display
can be realized. This integration allows manufacture of a lower-profile,
lightweight, and compact organic EL display at a low cost.

The above and many other objects, features and advantages of the present
invention will become manifest to those skilled in the art upon making
reference to the following detailed description and accompanying drawings
in which preferred embodiments incorporating the principles of the present
invention are shown by way of illustrative examples.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view showing the structure of a conventional
versatile organic EL display;

FIG. 6 is a perspective view showing an embodiment of the present
invention;

FIG. 7 is a plan view of the embodiment shown in FIG. 6;

FIG. 8 is a sectional view of the embodiment shown in FIG. 6;

FIG. 9 is an enlarged sectional view of FIG. 8;

FIG. 10 is a flow chart of the entire process of the manufacturing method
of the present invention;

FIG. 11 is a flow chart of an element formation process in the
manufacturing method of the present invention;

FIG. 12 is a flow chart of a cleaning process in the manufacturing method
of the present invention;

FIG. 13 is a graph showing the current-voltage characteristics of an
organic EL element, which is fabricated in accordance with the
manufacturing method of the present invention, in comparison with a
conventional element using a glass substrate;

FIG. 14 is a graph showing the luminance-current characteristics of the
organic EL element, which is fabricated in accordance with the
manufacturing method of the present invention, in comparison with the
conventional element using the glass substrate;

FIG. 15 is a perspective view showing the second embodiment of the present
invention;

FIG. 16 is an enlarged sectional view of the embodiment shown in FIG. 15;
and

FIG. 17 is an enlarged sectional view showing the third embodiment of the
present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Several preferred embodiments of the present invention will be described in
detail with reference to the accompanying drawings.

Referring to FIGS. 6 to 14, according to the best basic embodiment of the
present invention, metal electrodes 2 that form cathodes when completed
are formed on the upper surface of a board 1 called a printed wiring board
or PWB. An organic thin film 3 of organic EL elements is formed to cover
the metal electrodes 2, and thereafter transparent electrodes 4 that form
anodes are formed on the organic thin film 3.

Ordinarily, the metal electrodes 2 and transparent electrodes 4 are formed
in stripes to perpendicularly intersect each other through the organic
thin film 3. The respective metal electrodes 2 and transparent electrodes
4 are electrically connected to metal pads 5 that are insulated from each
other.

Each metal pad 5 is connected to a via interconnection 6 that connects the
upper and lower surfaces of the board 1, and is connected to a metal
interconnection 7 on the lower surface of the board 1. Circuit components
8, e.g., ICs such as latch-up circuits, drivers, or microcomputers, and
capacitors and resistors accompanying the ICs, are mounted (connected to
the metal interconnections 7) on the lower surface of the board 1 to form
drivers. In other words, organic EL elements are formed on the upper
surface of the board 1, and the drivers are formed on the lower surface of
the board 1. The organic EL elements and the drivers are integrated.

Although not shown in the structure shown in FIGS. 6 to 9, as the completed
structure, the organic EL elements are encapsulated with a transparent cap
or transparent resin. In the manufacturing process of FIG. 10, a resin
encapsulating step is performed.

The first embodiment of the present invention will be described.

As a practical example of the present invention, a commercially available
glass epoxy-based printed wiring board is used as a board 1. There is no
special reason for the use of the glass epoxy-based printed wiring board,
and basically any board material can be used. Note that since an organic
thin film is formed by vacuum deposition and transparent electrodes are
formed by sputtering, as will be described later, organic EL elements must
be able to withstand these processes. For example, no expansion of bubbles
in the board or no boiling of water content or solution in the board
should occur in vacuum. Preferably, since the durability of the organic EL
elements is impaired by water content and oxygen, it is better to choose a
material having a less water content and less oxygen.

The wiring pattern, including via interconnections 6 and metal pads 5, must
be order-made. The material of the interconnections (not shown in the
drawings) of the board 1 is Cu, and part of the interconnections may be
directly utilized as metal electrodes 2. In this embodiment, in order to
improve the performance of the organic EL elements, when forming the metal
electrodes 2, a thin film having a thickness of about 0.2 .mu.m to 1.5
.mu.m, containing Al as the major component and added with a small amount
of halogen element, e.g., Li, to about 1 weight % was formed on the entire
surface by vacuum deposition or sputtering, and were patterned into
stripes by photolithography. Other than this method, methods such as a
method of patterning the thin film into stripes without employing
photolithography but in accordance with a shadow mask method using a metal
mask can be employed.

The stripes are formed to have a pitch of 0.25 mm to 1.5 mm and a space
(gap) of about 0.01 mm to 0.1 mm. The pitch and space of the stripes
formed in practicing the present invention are not particularly
significant but can be arbitrary ones that compromise with the levels
required as a display and the micropatterning level. After the surfaces of
the metal electrodes 2 are treated with the cleaning process shown in FIG.
12, organic solvent treatment using IPA alcohol or the like is performed.
The resultant structure is cleaned with an ultrasonic flow of pure water,
and the surfaces of the metal electrodes 2 are lightly etched with a rare
HF solution or the like, and are cleaned with an ultrasonic flow of pure
water. After that, the resultant structure is sufficiently dried in an
inert atmosphere, e.g., nitrogen gas, and is set in a vacuum deposition
machine to form an organic thin film 3 of the next step shown in FIG. 10
by vacuum deposition. Although the organic thin film 3 is formed not to
cover the peripheral portion, as shown in FIG. 7, it is not patterned to
correspond to the individual metal electrodes 2 or transparent electrodes
4 (a so-called solid layer).

The thickness of the organic thin film 3 is about 100 nm to 300 nm, and to
be more specific, the organic thin film 3 has a multilayer structure
consisting of about 2 to 4 layers. FIG. 11 shows the manufacturing process
of an organic thin film having a four-layer structure. Opposite to the
case of an ordinary glass board, as shown in FIG. 11, an electron
transport layer is formed, an emission layer is formed on the electron
transport layer, and thereafter a hole transport layer and a hole
injection layer are formed. In the case of the embodiment, practically, an
aluminum quinoline complex Alq.sub.3 is used as an electron transport
material, Alq.sub.3 doped with quinacridone by co-deposition is used as an
electroluminescent material, diamine TPD is used as a hole transport
material, and copper phthalocyanine CUPC is used as a hole injection
material. The thickness of each layer is about 5 nm to 150 nm. To obtain
good characteristics, the thickness must be optimized. In practicing the
present invention, other organic EL materials can also be used.

After the organic thin film 3 is formed, the transparent electrodes 4 are
formed. After formation of the organic thin film 3 and before formation of
the transparent electrodes 4, the process is preferably performed without
breaking vacuum. Although the present invention can be practiced by
breaking vacuum as well, it is disadvantages in terms of characteristics
and formation of dark spots (dotted non-emission regions in an emission
region).

In the embodiment of the present invention, the vacuum deposition machine
and the magnetron sputtering device that forms the transparent electrodes
4 are connected to each other hermetically, and the transparent electrodes
4 can be continuously formed without breaking vacuum. Another point to be
observed in formation of the transparent electrodes 4 is to prevent
temperature increase in formation process as much as possible.

On the basis of the target described in Japanese Unexamined Patent
Publication No. 9-71860 indicated as the prior art and the film formation
condition recommended in this reference, the present invention is
particularly devised to suppress any increase in the temperature of the
board. The first measure is that a metal mask for patterning the
transparent electrodes 4 is inserted in the sputtering flow and is cooled
by heat transfer. Transparent electrode particles attaching to the surface
of the organic thin film 3 cannot form a film of a good quality unless
they have necessary heat or kinetic energy, and accordingly they cannot be
cooled. The metal mask plays no part in film formation. In the present
invention, the metal mask is cooled by heat transfer by using its holder
so that radiation heat from it is decreased.

The transparent electrodes 4 are formed to have a thickness of about 100 nm
to 300 nm in about 10 minutes. Ordinarily, as time passes, the temperature
of the board 1 increases. Regarding the quality of film formation of the
transparent electrodes 4, particularly its initial film formation state
when the transparent electrodes 4 constitute an interface with the organic
thin film 3 influences the characteristics of the organic EL elements.
Even if a transparent electrode film having a good quality is formed as
the temperature increases, it does not necessarily have improved
characteristics.

In the present invention, a mechanism for cooling the board 1 is added to
perform control so that the temperature of the board 1 does not increase.
Furthermore, part of the sputtering flow is ionized to control the energy
of the particles attaching to the surface of the organic thin film 3 by an
electric field. These necessary, not excessive control operations are
enabled, and the temperature of the board 1, to be more correctly the
temperature of the organic thin film 3, can be suppressed to a value that
the organic thin film 3 can withstand, which is estimated to be about
65.degree. C.

FIGS. 13 and 14 show the current-voltage characteristics and the
luminance-current characteristics of an organic EL element, which is
fabricated by practicing the present invention, in comparison with the
characteristics of the conventional organic EL element using a glass
substrate. Although the characteristics of the present invention are
slightly inferior to those of the conventional organic EL element,
characteristics with which an organic EL display can be set are obtained.

The second embodiment of the present invention will be described with
reference to FIGS. 15 and 16.

In the first embodiment, circuit components are mounted on the lower
surface of the board 1, and the organic EL elements and the drivers are
integrated through the board 1.

In the second embodiment, heat radiation fins 9 are mounted on the lower
surface of a board 1 through an adhesion layer 1a.

On the upper surface of the board 1, in the same manner as in the first
embodiment, an organic thin film 3 is sandwiched by striped metal
electrodes 2 and transparent electrodes 4, that perpendicularly intersect
each other, to constitute organic EL elements. Different from the first
embodiment, in the second embodiment, no via interconnections 6 are
formed, and the respective electrodes are connected to flexible leads
(FPC) at their peripheral portions on the upper surface side, so that they
are connected to the drivers formed on another board.

In this embodiment, the adhesion layer 1a thermally connects the board 1
and heat radiation fins 9 to each other well; thermal grease having a
thickness of about 10 .mu.m to 50 .mu.m is used as the adhesion layer 1a.
As the heat radiation fins 9, those made of Al material, commercially
available, and having an appropriate size, are used. Although the heat
radiation fins 9 are not particularly fixed to the board 1 with screws or
the like in this embodiment, it is preferable to fix the heat radiation
fins 9 with this method or other appropriate methods, as is often the
case.

The effect of mounting the heat radiation fins 9 resides in that heat of
about 1 W to 10 W which is generated upon emission of the organic EL
elements can be radiated promptly to suppress any temperature increase.
The material of the organic thin film 3 which is currently available lacks
heat resistance. According to the second embodiment, this drawback is
compensated for and the application field can be enlarged. As is indicated
in FIG. 11, heat generated by the organic thin film 3 is absorbed by the
metal electrodes 2 rather than by the transparent electrodes 4 as the
metal electrodes 2 have higher heat conductivity, is diffused, penetrates
the board 1, is transferred to the heat radiation fins 9 through the
adhesion layer 1a, and is radiated to the atmosphere from the heat
radiation fins 9. Although the temperature difference between the organic
thin film 3 and the atmosphere is small and the quantitative effect is
accordingly small, an effect of compensating for the lack of heat
resistance of the organic thin film 3 can be sufficiently obtained.

The third embodiment of the present invention will be described with
reference to FIG. 17.

In the second embodiment, the heat radiation fins 9 are mounted on the
lower surface of the board 1 to suppress any temperature increase of the
organic thin film 3. In the third embodiment, cooling elements such as
Peltier effect elements 10 are formed on metal interconnections 7 of a
board 1, thereby cooling a heated organic thin film 3.

When the Peltier effect elements 10 are used as the cooling elements, a
cooling effect can be obtained with a drive current; as the drive current
increases, the cooling effect advantageously increases. To form the
Peltier effect elements 10, the metal interconnections 7 of the board 1
are patterned in advance to correspond to the Peltier effect elements 10,
and the Peltier effect elements 10 are formed by deposition.

In formation of the Peltier effect elements 10, the shadow mask method
using, e.g., a metal mask, is employed for patterning. In the case of this
embodiment, the EL display device must be assembled so as not impair the
Peltier effect elements 10 during a surface treatment.